Infrared spectroscopy is a valuable, well-known tool for chemical characterization of gaseous, liquid and solid substances because compounds have distinct absorption “fingerprints” in the mid-infrared region, with absorption bands corresponding to vibrational energies of molecular bonds. In theory, infrared spectroscopy should be a very valuable tool for analyzing liquid samples for applications including, but not limited to: medical liquid analysis (blood, urine, saliva, etc.) for diagnostics or substance detection; industrial or food/beverage process control; and pollutant detection.
A major barrier to broader application of infrared spectroscopy to liquid samples has been the high inherent absorption of many liquids in the infrared. For example, water has strong infrared absorption, making analysis of aqueous solutions difficult. A number of tools have been developed to circumvent this issue, for example: the use of attenuated total reflection (ATR) prisms and other surface-grazing optical techniques; drying of samples before analysis; and the use of liquid-liquid extraction processes to transfer solutes from one liquid to another, more infrared-transparent liquid. Each of these introduces potential complexities and inaccuracies into measurements of liquids.
One approach to address some of these limitations is to use new and improved light sources in the infrared, including quantum cascade lasers (QCLs), that offer significantly higher power at specific wavelengths of interest than traditional globar (i.e. incandescent broadband thermal emitting) sources. This higher power potentially enables absorption measurements in thicker liquid samples, while maintaining sufficient power throughput to allow reasonable signal-to-noise for measurement of chemical concentrations in the sample. Measurements can then be performed with one or more wavelengths, with one or more “signal” wavelengths at absorption peaks of interest, and possibly wavelengths designed to provide reference or baseline levels (off-peak). Multiple wavelengths may be achieved using multiple lasers, or through the use of wavelength-tunable sources.
For detection of low concentrations of compounds in liquids, or subtle changes in chemical makeup, the incremental infrared absorption corresponding to concentrations of interest may be extremely small. Therefore even with higher power transmission, there remains the problem of detecting small absorption signals against a high background.
One approach to measure low concentrations in spectroscopy is the use of reference wavelengths. For example, sample transmission at the wavelength corresponding to an absorption peak of a substance of interest is measured, together with the transmission at two nearby wavelengths, one longer and one shorter. A “baseline” is then computed using the reference wavelength transmissions, and the transmission at the “peak” wavelength is divided by this baseline. This type of baseline adjustment can compensate for factors such as sample thickness, broad absorption by other compounds, and detector responsivity changes. In the case of broadband infrared sources, it also compensates—over a limited wavelength range—for changes in source output. For example, such referencing will drastically reduce effects from changes in temperature of a conventional blackbody thermal source. Indeed, this approach allows traditional Fourier-Transform Infrared (FTIR) instruments equipped with globar sources—or even using broadband radiation from synchrotron sources—to produce spectral data that may be locally baselined (in wavelength) to accurately determine chemical content.
Such baselining techniques, however, may be significantly less effective with infrared laser sources such as those that can deliver higher power to penetrate thicker liquid samples. Laser sources are inherently narrowband, resonant devices, rather than broadband emitters. Their output—power, wavelength, bandwidth, polarization and spatial beam properties—can be highly sensitive to device and operating conditions including current, temperature, aging, and feedback (from reflections). Moreover, any changes in these conditions may cause highly discontinuous changes in output. Moreover, these changes will not be consistent from one laser to another, or even from one wavelength to another in the case of a broadband or tunable laser. As a result, changes between illumination at the “peak” (absorption, of a target compound) wavelength and “reference” wavelengths may be very large compared to the incremental absorption from compounds of interest.
One method used for gaseous spectroscopy is the use of tunable lasers that scan through an absorption peak in a short time. This is the core concept behind tunable diode laser absorption spectroscopy (TDLAS) that is already used in commercial instruments. In gaseous samples, absorption peaks are typically very narrow (<<1 cm-1) and high. This means a very narrow tuning range may be used (often <1 cm-1 in wavenumber terms) to cover reference and peak wavelengths. This tuning may be performed quickly, and with minimum variation in laser conditions.
In liquid systems, on the other hand, absorption bands become far broader, with lower peak absorptions. This requires tunable systems to cover a broader range (>10 cm-1 or even >100 cm-1, for example) which is difficult to do consistently. For example, mode transitions within the laser may occur inconsistently, leading to sharp changes in power, wavelength, and other beam characteristics at the wavelengths of interest. Similarly, multiple discrete sources operating at wavelengths over the required range may individually vary in their emission characteristics over time and operating conditions, leading to apparent changes in “reference” and “peak” transmission and errors in reported chemical concentrations.
Furthermore, although it is possible to integrate reference power detectors that monitor laser power prior to the sample, such reference approaches frequently require beam splitting optics which will introduce new optical artifacts such as fringing into the system. Thus the power split off by these optics may be different from the power delivered to the sample as a result. In addition, such a reference channel will not account for optical effects within the sample and sample chamber—which can be particularly important in a coherent, laser-based system.